In the metal surface treatment art, chromium-based surface treating agents for chromate treatment and phosphate/chromate treatment were implemented and are widely utilized even at the present. However, the recent trend of environmental protection suggests an upcoming possibility to ban the chromium-based surface treating agents because of the toxicity and especially carcinogenicity inherent to chromium. There is a desire to have a surface treating agent that is free of chromium and that achieves high levels of metal adhesion and corrosion resistance comparable to chromate treating agents.
JP-A 8-73775 discloses an acidic surface treating agent comprising two distinct silane coupling agents. The acidic surface treating agent of this system, however, lacks corrosion resistance, which precludes the application thereof to metal members where good corrosion resistance and workability are required after the metal surface treatment.
JP-A 10-60315 discloses a surface treating agent for steel members comprising a silane coupling agent having a specific functional group capable of reacting with an aqueous emulsion. The corrosion resistance required in this patent is for relatively mild tests like wet tests. This surface treating agent is incomparably inferior to the rust-preventing agents which must clear stringent corrosion resistance.
There is a desire to have a non-chromate metal surface treating agent which in thin film form develops corrosion resistance and adhesion during working and which is useful as a coating agent for rust prevention.